The invention relates to methods and apparatus to produce uniformly-sized liquid droplets, particularly for use in deposition techniques using droplets of high-temperature liquids.
Liquid expelled as a jet stream flowing through a small (capillary) orifice under constant pressure tends to break up into droplets of non-uniform size and spacing. If ultrasonic vibrations of suitable frequency are induced in the liquid jet stream, pressure oscillations will propagate through the jet stream. Amplification of capillary waves in the liquid jet stream will cause the size and spacing of droplets formed in the expelled jet stream to become more uniform.
Vibrations may be induced in the liquid jet directly by tuning forks or other mechanical vibrators, or through forces induced by electromagnetic fields or piezoelectric driving elements that are coupled to the fluid jet stream through the walls of a containment vessel. Systems in which movement of the liquid jet stream through the orifice and application of the vibration are substantially uninterrupted are said to operate in a continuous mode.
Droplets may also be generated when a pressure pulse is applied to a static liquid in a vessel with one or more orifices. In such a system, no liquid flow occurs in the absence of a pressure pulse. However, when the liquid is subjected to a brief controlled pressure pulse, the resulting volumetric change in the fluid causes pressure/velocity transients to occur in the fluid. These transients in turn force a droplet of predictable size to issue from an orifice. Because a droplet is produced only when a pressure pulse is applied to the liquid, such a system is said to operate in a drop-on-demand mode.
In certain manufacturing processes, it is desirable to produce metal drops of a known, and perhaps uniform, size. Liquid metal systems operating in a continuous or drop-on-demand mode for droplet production have been described in U.S. Patents (e.g., U.S. Pat. Nos. 5,266,098 to Chun et al.; 5,229,016 to Hayes et al.; 3,683,212 to Zoltan; 4,527,717 to Emoto et al.; and 4,828,886 to Hieber, the disclosures of all of are incorporated herein by reference).
Droplet generators having magnetostrictive or piezoelectric driving elements (see Chun et al., Hiebet, and Zoltan) are generally limited to low melting-point metals and metal mixtures (70.degree.-200.degree. C.) in liquid metal applications because magnetostrictive or piezoelectric driving elements lose their responsive properties at temperatures greater than the Curie temperature of the material in question (about 350.degree. C. for typical piezoelectric materials such as lead zirconium titanate (PZT). Thus, droplet generators using piezoelectric or magnetostrictive driving elements have not been successful in systems using high-temperature solders or metals such as copper (melting point 1083.degree. C.).
Because droplet generators suitable for drop-on-demand or continuous service with high-temperature liquids have not been able to use piezoelectric driving elements, mechanical systems of valves or pistons have been favored instead. Mechanical generators can be made relatively heat tolerant; however, their mechanical linkages result in more limited frequency response than that obtainable with magnetostrictive or piezoelectric elements. Known high-temperature droplet generators operate at much lower frequencies and narrower bandwidths than those obtainable in low-temperature systems with piezoelectric driving elements. Thus, a need exists for broad-bandwidth droplet generators for use with high-temperature liquids.